4.2 Circuit Components

The various devices that are present within a circuit are commonly referred to as circuit components. For example, the battery, the switch, and the light bulb in previous examples are all circuit components. Many other components that perform different electrical functions are also available for the circuit designer to choose from. Of the many components that are available, some of these are useful for the design and implementation of digital circuits. To help us understand digital circuits we will take a closer look at some of the components that are commonly used in the design of digital circuits.     

The Resistor    

Resistors are circuit elements that restrict or reduce the flow of current in a circuit. Resistors come in various sizes and shapes. The most common form is in a cylindrical shape with connectors at either end. The resistance value in ohms (Ω) is indicated on the resistor using color bands. The colored bands are used because of insufficient space on the resistor to state its numerical value. Three colored bands are used to indicate the value of the resistance, and usually a fourth band to indicate the tolerance (the level of accuracy).  The color code is read with the tolerance band (gold, silver, or blank space) on the right.  The two left bands indicate the numerical value and the third band called the multiplier indicates the exponent of 10 that the numerical value has to be multiplied by. For example, a resistor with the color bands red-yellow-brown-gold has a value of 24x10¹ Ω (1^st digit red=2, 2^nd digit yellow=4, and multiplier brown=1).

Resistors could be combined in parallel and in series to achieve different values when a single resistor with the exact amount is not available (see Ohm’s law in section 4.1). For example, if 20 Ω is needed and only 5 Ω resistors are available then the 20 Ω resistance could be achieved using four 5 Ω resistors in series. Other arrangements could be used to get other values of resistance.

Table 4.1. Resistor colour codes.

Note: if the multiplier band is gold, then the multiplier is 10^-1. Similarly, if the multiplier band is silver, then the multiplier is 10^-2.

Semiconductors

Semiconductors materials that can be made to act as conductors or as insulators under different conditions. Semiconductors should not be thought of as poor conductors, or being in the middle of the conductor-insulator spectrum, but instead they behave as conductors or insulators under different conditions. Materials that show semiconductor properties are composed of elements from group 14 (IVA) of the periodic table. In this group of elements, silicon (Si) and germanium (Ge) are the most commonly used materials for semiconductor construction. For silicon to be used in semiconductor applications it has to be in a highly purified form, 99.9999% pure. The conducting properties of silicon can be altered by introducing impurities called dopants. The dopants are either from group 13(IIIA) or group 15(VA) of the periodic table of elements. Group 13 dopants are called p-type dopants and group 15 dopants are called n-type dopants. The n-type dopants from group 15 provide a surplus of electrons (negative effect) to the material, thus the term negative-type or n-type. Similarly, group 13 dopants provide a deficit of electrons (positive effect) to the material, thus the term positive type or p-type. When these p-type and n-type semiconductors are placed next to each other to form a junction, then different electrical properties are observed.

The Diode   

Diodes are semiconductor circuit elements that allow current flow in one direction only. In one direction the diode acts as a conductor, and in the other direction it acts as an insulator. The diode is constructed using a simple PN junction. When a P type and an N type semiconductor are placed next to each other as shown in figure 4.25, then current flow will be restricted to one direction only. If the power is connected such that the N side is connected to the positive of the power source and the P side is connected to the negative of the power source, then the charges will move away from the center and towards the sides.

When this happens a current flow cannot be established. If the polarity is reversed such that N is connected to the negative and P is connected to the positive, then the opposite effect takes place causing the electrons to move from the N to the P and causing a current flow.

A diode’s ability to perform this function is usually limited by the voltage. If a high enough voltage is applied to a diode then its insulating ability can break down, and the diode loses its ability to perform its function. Combinations of diodes can be used to perform the various logic functions. Since a diode is direction sensitive, the orientation of the diode within a circuit becomes important. The diode is marked with a band on one of its sides to indicate direction. The lead on the side of the band is the negative (-) and the lead on the other side is the (+).

The light-emitting Diode (LED)   

To identify the proper direction of the LED the leads of the LED are manufactured at with different lengths, as shown in figure 4.27. The shorter lead is the negative  which is also called the cathode, and the longer lead is the positive which is also called the anode. 

Another type of LED that is commonly used in computing related applications is the 7 segment LED. The 7 segment LED is a device that has 7 LED’s within it, and the LED’s are arranged in a manner that allows decimal digits to be displayed on it. This device is ideal for displaying output values in decimal. Each segment within the device operates as an independent LED allowing the device to display different combinations. The combinations that are visually similar to the various decimal digits are useful for displaying decimal output. The wiring on a 7 segment LED is setup as either common cathode or common anode. This means that all the cathodes are connected to a single pin on the device to produce a common cathode device and all the anodes are connected to a single pin to produce a common anode device. A common terminal arrangement is used to reduce the number of pins on the device. If the device is not wired this way, then it would require 14 pins (a cathode pin and an anode pin for each of the LED’s). Using a common terminal reduces the number of pins from 14 to 8, thus simplifying the wiring of the device.

Switches 

Switches are devices that activate and deactivate a circuit by opening and closing a circuit. Switches come in different styles and configurations, but all provide essentially the same functionality of opening and closing a circuit.  When a switch is said to be on, it enables a current flow in the circuit, and when it is said to be off it breaks or stops the current flow. This two-state function of a switch is similar to the two state nature of logic and binary numbers. With switches it is customary to denote “on” as 1 and “off” as 0. Due to the two state nature of on-off, true-false and binary, the ideas can be carried over from one into the other. Consider the following two circuits with two switches. When the switches are connected in parallel then the pattern that we get is the same as that of the logical operator OR.

Fig. 4.29. OR logic implementation using switches

When the switches are connected in series, then the pattern that we get is the same as that of the logical operator AND.

Fig. 4.30.  AND logic implementation using switches

From the above two circuits it can be seen that the switches can be used to implement logic operations, and this concept was the starting point for computing using electrical signals.

Transistors    

Transistors are circuit devices that can be thought of as switches that are turned on and off using electrical signals instead of a mechanical method. In the previous section we discovered how switches can be used to implement logic, but switches of the mechanical type are incapable of reacting to the logical result of the switch condition, because the input is mechanical and the output is electrical. With transistors the switching is controlled electrically, thus making the switching capable of reacting to the conditions of the logical result. The big advantage of a transistor is that both the input and the output are electrical, and it can be manufactured in physically small sizes. A transistor within a processor is much smaller than the cross sectional area of a strand of human hair. A typical personal computer has about a trillion (1,000,000,000,000) transistors with most of them located in the CPU, which usually occupies a small fraction of the space within the computer. 

Individual transistors come in different shapes and sizes, but in today’s market they are generally packaged in small black plastic cylindrical form with a section cut off. Usually the part number or id number is printed on the top or the cut off section of the packaging. All transistors have 3 leads called the emitter, base, and collector. The assignment of terminals called the pinout is established by facing the cut section of the cylinder. The terminal on the right is the collector, the base in the middle, and the emitter on the left as shown in figure 4.31.

Like diodes, transistors are also semiconductor devices. In the case of the transistor, a double junction is present as shown in figure 4.33.

The transistor in figure 4.33 is called a PNP transistor due to the internal arrangement of the junctions. Similarly a NPN arrangement could also be made. Each of the 3 leads of the transistor is connected to the three different sections of the transistor. In the PNP transistor a current flow is not present between the emitter and collector when they are connected to a voltage supply. A current begins to flow when a negative voltage is present at the base. Similarly, in a NPN transistor no current flow is present between the emitter and the collector when the base voltage is 0. Current flow begins when a positive base voltage is present. The voltage on the base terminal can be thought of as the switch that turns the current on and off. Using this idea of switching, two transistors could be used to perform logical operations of OR and AND by arranging them in parallel and in series (see arrangement and truth tables under switches). The transistor is the building block of devices that perform logic functions electrically called logic gates. The switching capability of a transistor is seen when a switch in a simple circuit is replaced with a transistor. In the case of the transistor, the switching function can be controlled electrically by controlling the voltage on base terminal.

The circuit in figure 4.34 shows a resistor, LED, and a transistor connected in series to a battery. When a current is present in the circuit then the LED will turn on. The circuit can be turned on by providing the base terminal of the transistor with a positive voltage. This can be done be connecting the base to the positive terminal of the battery through a resistor. A positive voltage is required because the transistor is of the NPN type.

Capacitors    

A capacitor is a device that are capable of storing electrical energy within it. A capacitor is constructed by placing two conductors close to each other, but without touching each other. A simple capacitor is produced by placing two conducting plates parallel to each other. When the parallel plates are connected to a voltage source, the charge from the source will be distributed over the two plates. Once a capacitor is charged the capacitor will retain its charge even if the voltage source is removed. Similarly, a charged capacitor can cause a current to flow if it is connected to a circuit forming a closed loop. The size of a capacitor is measured in Farads (F). The range of most capacitors is between a Pico farad (pF) or 10^-12 F and a microfarad (µF) 10^-6 F. Due to the fact that capacitors store electrical energy they must be handled with care, especially large size capacitors pose a significant electrical hazard.

Motors          

Electric motors are devices that use electrical energy to do mechanical work. Motors are found in various mechanical components of the computer. Disk drives, printers, and cooling fans are all examples of devices that use motors. Motors come in different shapes and sizes, and operate in different ways. As a general rule the larger the motor, the more electrical energy it will consume, and the more work it is capable of doing in a given amount of time. Most of us are familiar with a DC motor that rotates in a given direction when a voltage is applied to it, and rotates in the opposite direction when the polarity of the voltage source is changed. The problem with a simple DC motor is its lack of precision. It takes time for the motor to start turning, and it also takes time for it to come to a stop. These characteristics unfortunately are unsuitable for any application that requires precise mechanical operation. A class of electric motors called the stepper motor is of special significance to computing. It operates by turning fixed amounts or steps as the name suggests. A stepper motor can be used to make precise turns, which an AC or DC electric motor will not be able to do. Stepper motors are present in all types of disk drives because the disk that contains the information has to be turned and positioned precisely for it to be able to store data and retrieve data.

An understanding of the stepper motor operation is needed to effectively use this motor in computer applications. The moving part in the middle of a motor is called the rotor, and the stationary outer shell is called the stator, as shown in figure 4.35. The rotation of the rotor is made possible by the interaction of magnetic fields within the motor.

The stator has coils around the teeth that can create a magnetic field when a current is passed through it. The presence of a magnetic field on the stator teeth attracts the teeth of the rotor. For example, if the stator tooth at A has a current passing through it, then the rotor tooth and the stator tooth will be aligned as shown in figure 4.35. Removing the magnetic field in tooth A and creating a magnetic field in tooth B will cause the rotor to rotate clockwise by a small angle. Similarly, remove the magnetic field in A and creating a magnetic field in tooth D will cause the rotor to rotate counter clockwise by a small angle. The creation and removal of magnetic fields is done simply by controlling the current through the coils around each of the stator teeth. A sequence of ABCDABCD... would continuously rotate the rotor in a clockwise direction, and a sequence of ADCBADCB... would continuously rotate the motor counter clockwise.  A different sequence of input current would not cause continuous rotation of the rotor. For example, an input sequence of ABADABAD... would cause the motor to oscillate back and forth.  A random sequence of inputs to a stepper motor will not produce any useful output. This ability of the stepper motor to rotate in both direction and change directions relatively abruptly is ideal for computing application because it can be used to position mechanical devices precisely, like a disk drive head. The precise mechanical nature of the stepper motor also makes it an ideal tool for robotic applications where precision is very important. Unlike most common motors that have two leads that connect to the voltage source, the stepper motor will have several leads coming out of it. Each coil around a tooth in the stator needs to be wired and controlled independently, therefore several leads will be present to control each coil independently.

Voltage regulators

A voltage regulator is a device that provides a regulated constant voltage source using a variable input voltage. These devices are ideal for digital circuits because digital circuits operate at fixed voltages and tend to be sensitive to voltage fluctuations. Voltage regulators come with different specifications for input voltage, output voltage, and current. The 7805 Voltage regulator is ideal for digital circuits based on the 5V standard, because it provides a constant output voltage of 5V for input voltage in the range of 8-18V. This is ideally suited for use with a 9V battery or four AA 1.5V batteries in series for a total voltage value of 6V. Note: rechargeable AA batteries are 1.2V and an arrangement of four in series will produce only 4.8V which is not sufficient for 5V digital circuits. The metal plate that is usually found on voltage regulators is used to dissipate the excess heat generated by the voltage regulator. The pin assignment or pinout is shown in figure 4.37.

Integrated circuits (IC)

Integrated Circuits (IC), commonly called chips, are circuits with several components that are packaged into a single physical unit or object. Usually IC’s are designed to perform a given function. The job performed by an IC can vary from a simple task to a very complex task. An example of a simple IC would be one that performs logic gate functions, and an example of a complex IC would be a CPU. The number of output terminals, commonly called pins, found on an IC usually indicates the complexity of the IC. For example, the 7486 IC has 4 gates and has 14 pins, and an Intel 80486 CPU IC has 64 pins. IC technologies vary with the scale of integration, and are categorized as follows.

The assignment of the pin function on an IC is called the pinout of the IC. Knowing the pinout of the IC is essential before it can be used in any circuit. Two ICs that look the same and have the same number of pins do not necessarily have the same function.

ICs come in different shapes and sizes, but most IC’s are commonly packaged or made into one of 3 types of physical arrangements, as shown in table 4.3.

Table 4.3. IC package configurations

In each type of physical packaging it is important to distinguish the orientation of the IC and its corresponding pinout. ICs are manufactured with a notch or marker on one of its sides to provide the proper orientation for the IC’s pinout. If the IC is viewed from the top and the notch is on the left then the pin assignment begins with pin 1 on the bottom left and increasing in a counter clockwise direction as shown in figure 4.39 for a DIP packaging.

IC’s that are used in digital circuits usually contain a large number of transistors. These types of ICs are manufactured by creating transistors at the microscopic level. For example, a CPU is manufactured by producing trillions of transistors on a thin sheet of silicon called a silicon wafer. The wafer is then processed using etching and depositing processes to create a circuit with components at the microscopic level. A typical CPU will contain several layers of components with component counts in the trillions.

Sensors

Sensors are circuit devices that are used to detect environmental conditions. These devices are used to collect environmental information which can then be used by a processor to analyze and respond to as needed. For example, a switch can be used as a tactile (touch) sensor. Imagine a switch that turns on when pushed. If this switch is placed at the front of a robot, then when the robot hits a wall it can be used to detect the wall and stop the forward motion of the robot. Therefore, in this application the switch is acting as a sensor.

Sensors of different types are available for use in different applications. For example, the oxygen sensor in a car engine measures the oxygen level in the car exhaust and sends it to the engine computer (embedded system) so that adjustments to the engine are made to operate the engine in an optimal manner. This results in better engine performance and reduced pollution.

    

Fig 4.40. Examples of sensor applications

Explore Further: Smartphone Sensors  Do an online search to determine the type of sensors that are used on a smartphone. Using your research create a table with the type of sensor and how it is used in a smartphone. Develop an idea for a sensor that is currently not available on a smartphone. Use your imagination.

4.2 Practice Questions 1.     Determine the resistance of resistors with the following color bands.         a. blue green black gold                             b. red yellow orange gold         c. gold green black brown                         d. silver red orange blue 2.     List five circuit components that are commonly used in digital circuits. 3.     How is a light bulb different from an LED? 4.     Explain how two switches can be used to create the OR logical function. 5.     What is a transistor and how can it be used to perform logical operations? 6.     What is an IC and how is it different from other circuit components? 7.     Two IC’s have the same colour, shape, and same number of pins. Will the two IC’s perform the same function? Explain. 8.     Determine the diode circuit that will function as a 3 input AND gate. 9.     Explain the difference between an N-type and a P-type semiconductor. 10.   Determine the arrangement of transistors that will function as an OR gate. 11.   Explain the difference between a regular DC motor and a stepper motor. 12.   Explain how the pin assignment on an IC is determined.